Scala collection filter by type

Question

im new to scala and ran into the following problem:

I want to get a subcollection of an existing collection that only contains elements of a specific type. The following works:

class C(val name : String)
class D(name : String) extends C(name) { }

val collection = Set[C](new C("C1"),new D("D1"),new C("C2"),new D("D2"))
collection.collect{case d : D => d}.size must be === 2 // works

But when i try to extend the collection classes with a method "onlyInstancesOf[Type]" this does not work. First my implementation:

object Collection {
    implicit def extendScalaCollection[E](coll : Traversable[E]) = new CollectionExtension[E](coll)
}

class CollectionExtension[E](coll : Traversable[E]) {

    def onlyInstancesOf[SpecialE <: E] : Traversable[SpecialE] = {
        coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
    }
}

So when i use this extension and execute:

collection.onlyInstancesOf[D].size must be === 2

I get an error that .size returned 4 and not 2. Also, i checked, the result actually contains C1 and C2 though it should not.

When i do:

collection.onlyInstancesOf[D].foreach(e => println(e.name))

I get the exception:

java.lang.ClassCastException: CollectionsSpec$$anonfun$1$C$1 cannot be cast to CollectionsSpec$$anonfun$1$D$1

So obviously the resulting set still contains the elements that should have been filtered out.

I dont get why this happens, can anyone explain this?

Edit: Scala: Scala code runner version 2.8.0.final

Answer 1

As others have pointed out, manifests can rescue you. Here's an example of how, restricting ourselves to non-primitives, and assuming we don't want to store manifests in our collections but instead use reflection on the spot to figure things out:

class CollectionExtension[E <: AnyRef](coll : Traversable[E]) {
  def onlyInstancesOf[SpecialE <: E](implicit mf : Manifest[SpecialE]) : Traversable[SpecialE] = {
    coll.collect({
      case special if mf.erasure.isAssignableFrom(special.getClass) => special
    }).asInstanceOf[Traversable[SpecialE]]
  }
}

and here it is in action:

scala> val ce = new CollectionExtension(List(Some(1),Some(5),"This","Fox")) 
ce: CollectionExtension[java.lang.Object] = CollectionExtension@1b3d4787

scala> val opts = ce.onlyInstancesOf[Some[_]]
opts: Traversable[Some[_]] = List(Some(1), Some(5))

scala> val strings = ce.onlyInstancesOf[String] 
strings: Traversable[String] = List(This, Fox)

Answer 2

Pay attention to the compiler warnings, and add -unchecked your scala command line options.

M:\>scala -unchecked
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21)
.
Type in expressions to have them evaluated.
Type :help for more information.

scala> class CollectionExtension[E](coll : Traversable[E]) {
     |
     |     def onlyInstancesOf[SpecialE <: E] : Traversable[SpecialE] = {
     |         coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
     |     }
     | }
<console>:8: warning: abstract type SpecialE in type pattern SpecialE is unchecked since it is eliminated by erasure
               coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
                                            ^
defined class CollectionExtension

The warning means that the best the compiler can do is equivalent to:

coll.collect({case special : AnyRef => special}).asInstanceOf[Traversable[_]]

For a more detailed explanation of type erasure, and ways you can work around it with Manifests, see:

https://stackoverflow.com/questions/tagged/type-erasure+scala